Circuit arrangement for generating a variable magnetic field

ABSTRACT

A circuit arrangement for generating a magnetic field periodically varying with time and being adjustable in amplitude, in which an extreme value of the field must be adjusted at a fixed level. The field is generated by a coil which is wound on a core of magnetic material. An auxiliary coil is wound on the same core through which coil a direct current adjustable by means of a potentiometer circuit flows in such a manner that the extreme value of the resultant field is adjusted at a fixed level. One embodiment of the circuit arrangement includes two mechanically coupled potentiometers. One possibility of use thereof is the convergence in a colour picture display apparatus in which the field is the combination of a parabolic and a sawtooth component. In the case of the horizontal convergence, the auxiliary coil may be the vertical convergence coil.

ilnited States Patent a. [191 De Vries et a1.

[ May 28, 1974 CIRCUIT ARRANGEMENT FOR GENERATING A VARIABLE MAGNETIC FlELD [73] Assignee: S. Philips Corporation, New

York, N.Y.

22 Filed: July 9,1973

21 Appl. No.: 377,798

Related U.S. Application Data [63] Continuation of Ser. No. 117,345, Feb. 22, 1971,

abandoned.

[52] U.S. Cl 315/13 C [51] Int. Cl. H0lj 29/70 [58] Field of Search 315/13 C, 13 CG, 27 GD, 315/27 XY, 27 R, 27 TD, 13 R 6/1971 Siegel ..315 1 3c Primary ExaminerMaynard R. Wilbur Assistant ExaminerJ. M. Potenza Attorney, Agent, or Firm-Frank R. Trifari; Henry I.

Steckler [5 7 ABSTRACT A circuit arrangement for generating a magnetic field periodically varying with time and being adjustable in amplitude, in which an extreme value of the field must be adjusted at a fixed level. The field is generated by a coil which is wound on a core of magnetic material. An auxiliary coil is wound on the same core through which coil a direct current adjustable by means of a potentiometer circuit flows in such a manner that the extreme value of the resultant field is adjusted at a fixed level. One embodiment of the circuit-arrangement includes two mechanically coupled potentiometers. One possibility of use thereof is the convergence in a colour picture display apparatus in which the field is the combination of a parabolic and a sawtooth component. In the case of the horizontal convergence, the auxiliary coil may be the vertical convergence coil.

5 Claims, 5 Drawing Figures PNENIEDwza m4 SHEEIIWZ IN VE N TORS This is a continuation of application Ser. No. 1 17,345, filed Feb. 22, I971 now abandoned.

The invention relates to a circuit arrangement including a coil for generating a magnetic field varying with time and being adjustable in amplitude, which coil is wound on a core of magnetic material and through which an alternating current flows, an auxiliary coil being wound on the same core in order to adjust an extreme value of the field at a fixed level and both coils having the same winding sense.

Such a circuit arrangement is described in US. Pat. No. 3,714,495, filed June 4, 1970, and also in the publication Philips application Information No. 261 Circuits for l l Color Television Picture Tube pages 16 to 18. The publication relates to a circuit arrangement for the so-called horizontal radial dynamic convergence of the electrons beams in a colour picture display tube of the shadow mask type in which the current flowing through each convergence coil is driven through a capacitor by an amplifier formed as a voltage source. Since this current must be mainly parabolic, a sawtooth voltage of line frequency must be applied in the first instance between the terminals of the coil. The mean value of the current is zero, so that the maximum value which is reached in the middle ofthe line scan period is not zero and is dependent on the adjustable peak-to-peak amplitude of the parabola. However; it is desired that the induction field generated by the current is zero at that instant because the beam then scans the central vertical line on the screen of the display tube during which no horizontal dynamic convergence is to occur. The quoted circuit arrangement is therefore provided with a compensation coil through which a direct current flows whose intensity is equal to the means value of the positive portion of the parabolic convergence current. This coil is wound on the same core as the convergence coil and has a number of turns such that the resultant induction field is always zero in the middle of the line scan period.

As is known. it is possible that the convergence current must also include a sawtooth component. For this purpose a pulsatory voltage is applied to that terminal of the convergence coil which is not connected to the amplifier, which voltage has an amplitude required for an average tube. To be able to adjust the sawtooth current as desired on either side of this means value, the amplifier is controlled by an adjustable pulsatory voltage having a positive or negative polarity.

As long as the above-mentioned pulsatory control voltage remains small, that is to say, as long as the sawtooth current component is to be considered a kind of small correction, the circuit arrangement according to the quoted publication operates satisfactorily. However. it has been found that it is possible that the sawtooth current component still must have a large amplitude. The detrimental result thereof is that the direct current field no longer compensates the alternating current field in the middle of the scan because the mean value of the convergence current has changed. ln fact. the ratio of the numbers'of turns on the convergence and compensation coils applied to the pure parabolic shape and therefore does not correspond tothe new mean value. Then a shift in convergence occurs in the vicinity of the central vertical line.

- 2 An object of the present invention is to obviate the drawback described. In this case the compensation coil is no longer incorporated in the amplifier and the circuit arrangement according to the invention is characterized in that a potentiometer circuit connected to a direct voltage source is provided, which circuit applies a direct current to the auxiliary coil the intensity of which current is substantially proportional to the peakto-peak amplitude of the alternating current.

ln order that the invention may be readily carried into effect, some embodiments thereof will now be described in detail by way of example with reference to the accompanying diagrammatic drawings, in which:

FIGS. 1, 3a and 3b show embodiments of the circuit arrangement according to the invention, and

H68. 2a and 2b show two current waveforms which occur in the circuit arrangement according to H6. 1.

ln P16. 1, the reference numeral 1 denotes an input terminal to which a parabolic voltage of line frequency originating from the line deflection circuit in a color picture display apparatus is applied. Terminal 1 is, for example, a terminal of the capacitor for the so-called S-correction. Two potentiometers 3 and 4 are connected in parallel with a winding 2 of the line output transformer. Terminal land the wiper on potentiometer 4 are connected by means of a capacitor 5 of low capacitance and a resistor 6, capacitor 5 being shunted by a resistor and resistor 6 being shunted by the series arrangement of a potentiometer'7 and a resistor 7. The

, wiper on potentiometer 7 is connected to the wiper on a further potentiometer 8 whose ends are connected through two resistors of equal value to the ends of the secondary winding on a transformer 9. One end 'of the primary winding is connected to earth and the other end is connected to the wiper on potentiometer 3, while a central tap of the secondary winding is connected to the wiper on potentiometer 4.

One end of potentiometer 8 is connected to the base of a pnp-transistor 10, whose emitter drives a second pnp-transistor 11, while the collectors of bothtransistors are connected to a negative supply line -V The series arrangement of three resistors l2, l3 and.14 is provided-between this line -V and a positive supply line +V resistor 13 being shunted by a capacitor 15 and resistors 12 and 14 having equal values. The junction of resistors 13 and 14 is connected to the base of transistor 10 and the junction of resistors 12 and 13 is connected to the base of an npn-transistor 16, whose emitter drives a second npn-transistor 17, while the collectors of both transistors 16 and 17 are connected to line +V,,,. The emitters of transistors 11 and 17 are interconnected and, via a capacitor 18, they pass a current through the coil 19 for the horizontal dynamic convergence of the red electron beam. The series network 20 of a capacitor and a resistor is arranged between the junction of capacitor 18 and 19 and earth.

Correspondingly, the other end of potentiometer 8 is connected to the base of a transistor 10' and corresponding elements 11' to 20 are provided which ensure' the convergence of the green electron beam. The free end of coil 19 and coil 19' is connected to a terminal 21 to which a pulsatory voltage originating from the line deflection circuit is applied.

The parabolic voltage present on terminal 1 is differentiated by the network consisting of capacitor 5 and resistor 6. so that a sawtooth voltage increasing with time is produced at the junction of these elements. Resistor 13 and capacitor 15 constitute a direct voltage source which corresponds to the sum of the baseemitter junction voltages of transistors 10, ll, 16 and 17. Resistor 13 and capacitor 15 may be replaced by diodes in known manner. Since no direct voltages are applied to the input terminals of the circuit arrangement and since both amplifiers are fed by two supply voltages +V,, and V which are equal in absolute value, for example, +25V and 25\ these amplifiers automatically operate in class B. As a result the convergence current is supplied alternately by transistors 17 and 17 and by transistors 11 and 11'. Due to the class- B adjustment the convergence current is proportional to the control voltage of transistors 17 and 11 and 17 andll. These transistors may alternately be adjusted in class AB so as to compensate the non-linearity of the characteristics thereof. Since the line repetition frequency is comparatively high, the ohmic resistance of coil 19 is negligible. Consequently, a current flows through the coil, which current, as a function of time,-

is the integral of the control voltage of the amplifier, which is a parabolic current when this control voltage is purely sawtooth shaped. The amplitudes of the parabolic convergencecurrents for red and for green may be adjusted by means of potentiometer 7 in the same direction and by means of potentiometer 8 in the opposite direction. Potentiometer 7 therefore serves for the R-G-parahola amplitude control and potentiometer 8 serves for the R-G-parabola balance control.

It is evident thata sawtooth voltage on'terminal 1 could have been taken as a starting point so that the differentiating network 5, 6' could have been omitted.

However, there is no sawtooth voltage available in the display apparatus. which voltage is proportional to the line deflection current. This proportionality is necessary because the line deflection current is not constant, but varies as a result of, for example, the correction for the so-called East-West pin-cushion distortion. It is alternatively possible for this current to vary as a result of variations in the power main supply voltage. A known step to stabilise the width of the picture displayed in this case is in fact to vary the EHT generated in the line deflection circuit and required for the acceleration anode of the picture display tube by the same percentage as the mains supply voltage and to vary the deflection current by halfthe percentage. The variation in the convergence current must therefore be proportional to that of the deflection current in order that the convergence is satisfactory at any area on the screen of the picture display tube.

The pulsatory voltage present on terminal 21 produces a sawtooth current through coil 19, which current is also proportional to the line deflection current and which also flows through transistors 11 and 17, but without producing a voltage at the low impedance junction the emitters of these transistors so that the power supplied by these transistors is not increased. This power is reduced by means of the series network 20 consisting of a resistor of comparatively low value and a capacitor so that a pulsatory current is added to the current flowing through the transistors. A similar result may alternatively be achieved with series arrangements of a coil and a capacitor. The voltage on terminal 21 has an amplitude such that the amplitude of the sawtooth current is approximately equal to the value which is necessary for the satisfactory convergence of an average tube. However, it must be possible to adjust the sawtooth component on either side of this mean value due to the tolerances in the various components'of the picture display apparatus. For this reason a pulsatory voltage originating from winding 2 is added to the sawtooth voltage at the input of the amplifiers. The pulsatory voltage may be adjusted in amplitude and polarity for both amplifiers by means of potentiometer 4, while potentiometer 3 provides the possibility of increasing the control voltage of one amplifier in the positivedire'ction and of increasing that of the other amplifier in the negative direction by the same amount. Potentiometer 4 therefore serves for the *R-G-sawtooth amplitude" control and potentiometer 3 serves for the R-G- sawtooth balance" control. Due to the symmetry of the circuit arrangement the pulsatory and sawtooth control voltages hardlyinfluence each other.

Since the current flowing through the convergence 'coil 19 cannot comprise a DC component, the waveform thereof is situated about the zero line as a function of time, in such a manner that its mean value is zero,.the positive part being provided by transistor 17 .and the negative part being provided by transistor 11.

namic convergence is zero in the centre of the horizontal lines in order that the static anddynamic adjust ments are independent of each other.

The invention is based on the recognition of the fact that the use of an auxiliary coil makes it possible to render the dynamic convergence field zero in the centre of the horizontal lines independently of the peak-to-peak amplitude of the current flowing through coil 19. Furthermore, the invention is based on the recognition of the fact that this auxiliary coil may be a coil which is already present in the picture display apparatus and that the current flowing therethrough must always be proportional to the peak-to-peak amplitude of the parabolic current component, for the sawtooth component of the current flowing through coil 19 is zero in the middle of the line scan period.

The coils 22 and 22 for the vertical convergence are wound on the same core of magnetic material as that on which coils 19 and 19' for the horizontal convergence are wound, said coils 22 and 22' constituting the collector load of two final transistors 23 and 23' which form part of two AC amplifiers not further shown in H6. 1. Coils 22 and 22' also operate as auxiliary coils for clamping the convergence field of line frequency. Coil 22 serves for the red" convergence and coil 22 serves for the green convergence. A resistor 24, a potentiometer 25 and a resistor 24' are arranged in series between the collectors of transistors 23 and 23, resistors 24 and 24 having equal values. The wiper and one end of the further potentiometer 26 are connected together and to the wiper on potentiometer 25, while the other end of potentiometer 26 is connected to a resistor 27. The free ends of coils 22 and 22' are connected to supply line +V and that of resistor 27 is connected to supply line V The wiper on potentiometer 26 is mechanically coupled to the wiper on potentiometer 7 and the wiper on potentiometer 25 is mechanically coupled to that on potentiometer 8.

Coils l9 and 22 and 19 and 22 are wound in such 'a sense that an increase in the induction field generated by the current flowing in one coil corresponds to an increase in the induction field generated by the current flowing in the other coil. Since a field is proportional to the current which flows through the relevant coil, the field generated by coils 22 and 22 constitutes the combination of a direct current field and of a field frequency field. The condition therefore is that the magnetic material used does not substantially have any hysteresis and that the induction field is always proportional to the magnetic field.

When in FIG. I the wiper on potentiometer 7 is moved upwards, the amplitude of the parabolic current component flowing through coils l9 and 19 increases and the negative maximum value 1' thereof increases. This value is an increasing and substantially linear function of the resistive value I of the part of potentiometer 7 between its wiper and its junction with resistor 7 and therefore of the angle of rotation d) of potentiometer 7. Simultaneously the resistive value 'x of potentiometer 26 decreases so that the direct current flowing through coils 22 and 22' and consequently the direct current field generated by these coils increases; The said field thus increases with the angle of rotation (15, but this variation is not linear. In fact, as a function of 5 there applies that wherein .r and 115 represent the maximum values of .r and d and in which potentiometer 26 is assumed to be linear. If R is the resistive value of the rest of the path located between lines +V,,, and --V,, in which potcntiomctcr 26 is incorporated, the following current flows:

This function is plotted in FIG. 2a.

The maximum i to be compensated is proportional to the some of I and of the resistive value R of 7':

wherein R is the total value of potentiometer 7, while k is a proportionality constant. In this expression y is proportional to angle of rotation 4) when potentiometer 7 is also linear so that This function is plotted in FIG. 2b.

The fields are now proportional to I and i and both to be constant since the field period is many times longer than the line period, so that the field frequency field does not exert any influence on the instant in the line scan period when the resultant line frequency field is zero.

If thewiper on potentiometer 8 is moved, the parabolic, current component through one of the two coils l9 and 19, for example, 19 is increased and is decreased by the same amount in the other coil. Simultaneously the direct current flowing through one of the two coils 22 and 22', in this case coil 22, is increased and decreased in the other coil. These variations are, substantially linear functions of the angle of rotation of the potentiometers, which functions may be made identical by the choice of the different resistors. A satisfactory proportionality between the maximum value of the convergence current for each colour and the compensation current is always ensured in this manner.

In the embodiment described the vertical convergence coil operates as an'auxiliary coil for the purpose of compensating the variation of the maximum value of the parabolic current component as a function of its amplitude. It is evident that a different coil may be used for the same purpose. An additional coil may be wound, for example, on the same core of magnetic material on which coil l9.is also wound, which provides an extra degree of freedom when-proportioning the circuit arrangement, namely the choice of the inductance of the additional coil. It is alternatively possible to use the coils for the static convergence.

FIG. 3a shows a simplified circuit diagram of a further embodiment-of the arrangement according to the fields can be plotted as a function of d), the curves ob- 2 tained having the same shape as the curves of FIGS. 2a and b. The resistors of both circuit arrangements may be given values such that the curves obtained intersect at two points and are elsewhere located together as closely as possible. In this manner it may be achieved that the field generated by direct current I is substantially equal for all positions of potentiometers 7 and 26 to the absolute value of the field generated by maximum current i. A still better approximation may be obtained by using non-linear potentiometers. Under these circumstances the resultant induction field is always zero in the middle of the line scan period. It is to be noted that the field frequency field generated by coils 22 and 22' during a line scan period may be considered invention. For the sake of simplicity, only one amplifier, the one for the red-. convergence, is shown while coil 22 is a separate and not the vertical convergence coil. In this embodiment potentiometer 7 takes over the functions of potentiometers 7 and 26 of FIG. 1, namely the adjustment of the amplitudes of the sawtooth control voltage of transistors 11 and 17 and of the direct current flowing through auxiliary coil 22. A resistor 28 is arranged between line +V,, and potentiometer 7 and a resistor 29 is arranged between the wiper on potentiometer 7 and coil 22, while the free ends of potentiometer 7 and of coil 22 are connected to earth. The junction of resistor 29 and coil 22 is decoupled for components of line frequency by means of a capacitor 30. As is shown in the embodiment of FIG. 1 resistors 7, 28 and 29 may be given values in order that the variation of the direct current flowing through auxiliary coil 22 is substantially proportional to the amplitude of the 7 parabolic current component;

A'better separation between the three functions of the control circuit, which functions are the supply of the sawtooth and pulsatory control voltages as well as of the compensating direct current, is obtained in the embodiment of FIG. 3b. In this embodiment the junction of resistor 29 and capacitor 30 controls the base of a transistor 31 whose collector lead incorporates auxiliary coil 22, which transistor 31 may form part of the amplifier for the vertical convergence if auxiliary coil 22 is the coil for the vertical convergence.

The arrangements described ensure the horizontal convergence of the red and green electron beams. However, it will be evident that a similar arrangement may also be used for the blue convergence. A similar compensation is in principle also possible for the vertical convergence in which the relevant coils must have larger dimensions because the field repetition frequency is much smaller than the line repetition frequency.

Transistors are used as amplifier elements in the embodiments described. The principle of the invention is not affected when other known amplifier elements are used such as, for example, electron valves or field effect transistors. Alternatively, waveforms other than the one described may occur. It is also to be noted that the field of application of the circuit arrangement according to the invention need not be limited to convergence circuits. but may be used in all cases where an extreme value of a periodically varying magnetic field is to be fixed at a given level.

What is claimed is:

1. A circuit comprising a magnetic core; a main coil wound on said core; means for applying to said core an alternating current having parabolic and sawtooth components, each having a given peak to peak amplitude respectively, whereby said coil and core generate a magnetic field having a given peak to peak amplitude; an auxiliary coil wound on said core; and means including a first potentiometer for supplying to said auxiliary coil a direct current having a value substantially proportional only to said alternating parabolic current component peak to peak amplitude, whereby an extreme value of said magnetic field is clamped to a selected level.

2. A circuit as claimed in claim 1 further comprising means including a second potentiometer for adjusting the amplitude of said parabolic component, the wipers of said first and second potentiometers being mechanically coupled.

3. A circuit as claimed in claim 1 further comprising a color display tube having at least two electron beams, and at least two coil means for converging said beams respectively.

4. A circuit as claimed in claim 3 further comprising potentiometer means for adjusting the current in both of said convergence coil means in opposite directions with respect to amplitude changes of said currents in said convergence coil means, two series coupled auxiliary coils, one free terminal of each of said coils being coupled together and to said direct voltage source, a further potentiometer mechanically coupled to said opposite direction potentiometer, a pair of resistors coupled between said further potentiometer and said auxiliary coils respectively, and a variable resistor mechanically coupled to said same direction potentiometer and coupled between said further potentiometer and said direct current source.

5. A circuit as claimed in claim 4 wherein said main and auxiliary coils comprises horizontal and vertical convergence coils respectively.

73 3 2; UNITED STATES PATENT OFFICE CERTHHQATE OF CORRECTION Patent No. 3,813,573 Dated May 28, 1974 Inv nc fls) JAN LOLKE DE VRIES ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shownbelow:

r. I IN THE TITLE PAGE below "Appl. No, insert Foreign Application Priority Data March 7, 1970 Netherlands .7003283 Signed and sealed this 8th day of October 1974.

(SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents MCCOY M, GIBSON JR Attesting Officer mg UNITED STATES PATENT OFFICE v v CERTEFHQATE OF CORRECTION Patent No. 3,813,573 Dated May 28, 1974 In nt fls) JAN LQLKE DE VRIES ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown'below:

I. IN THE TITLE PAGE below "Appl. Noo insert Foreign Application Priority Data March 7, 1970 Nether lands' ..7003283 Signed and sealed this 8th day of October 1974.

Attest:

C. MARSHALL DANN Commissioner of Patents MCCOY M, GIBSON JRO Attesting Officer 

1. A circuit comprising a magnetic core; a main cOil wound on said core; means for applying to said core an alternating current having parabolic and sawtooth components, each having a given peak to peak amplitude respectively, whereby said coil and core generate a magnetic field having a given peak to peak amplitude; an auxiliary coil wound on said core; and means including a first potentiometer for supplying to said auxiliary coil a direct current having a value substantially proportional only to said alternating parabolic current component peak to peak amplitude, whereby an extreme value of said magnetic field is clamped to a selected level.
 2. A circuit as claimed in claim 1 further comprising means including a second potentiometer for adjusting the amplitude of said parabolic component, the wipers of said first and second potentiometers being mechanically coupled.
 3. A circuit as claimed in claim 1 further comprising a color display tube having at least two electron beams, and at least two coil means for converging said beams respectively.
 4. A circuit as claimed in claim 3 further comprising potentiometer means for adjusting the current in both of said convergence coil means in opposite directions with respect to amplitude changes of said currents in said convergence coil means, two series coupled auxiliary coils, one free terminal of each of said coils being coupled together and to said direct voltage source, a further potentiometer mechanically coupled to said opposite direction potentiometer, a pair of resistors coupled between said further potentiometer and said auxiliary coils respectively, and a variable resistor mechanically coupled to said same direction potentiometer and coupled between said further potentiometer and said direct current source.
 5. A circuit as claimed in claim 4 wherein said main and auxiliary coils comprises horizontal and vertical convergence coils respectively. 